The answer on the eveness of a symetrical curve can only be gotten at after the curve is chosen. What I mean is that I would do it differently for an elipse, than a simple arch, than a segmented arch, than a compound arch. Let me know which you choose and I'll offer appropriate advice. The way I do it most often is a bow arch referenced to 3 points in space. But when I did an ellipse, that was a lot more complex. A compound arch is a middle ground. Between the two in terms of difficulty. All are viable however.
As to the more general comment on fairing the piece off the bandaw, I use a spokeshave. Most probably use a router table and pattern bit.
Shawn
"no trees were harmed in the creation of this message, however some electrons were temporarily inconvenienced."
"I resent having to use my brain to do your thinking"